1. Field of the Invention
This invention relates to an apparatus for causing recording liquid to be discharged, for example, from a full line recording head of recording medium width to a recording medium being conveyed by feeding means, thereby accomplishing the recording of information such as characters and images.
2. Description of the Prior Art
A serial type recording apparatus and a full line type recording apparatus are known as ink jet recording apparatuses of this kind for causing recording liquid to be discharged from a recording head to thereby accomplish the recording of information such as characters and images.
The serial type recording apparatus is of a form in which recording is effected by a recording head carried on a carriage while the carriage is moved along a platen holding a recording medium and sheets are fed in a direction perpendicular to the direction of movement of the carriage, and the full line type recording apparatus is of a form in which a recording head is provided with ink discharge ports disposed over the recording width in the main scanning direction and such a recording medium is moved in a sub-scanning direction relative to a recording head to thereby effect recording.
There has also been proposed an apparatus which assumes one of the above-described forms and yet is constructed so that by a plurality of recording heads being disposed, not only monochromatic recording but also color recording can be accomplished.
Referring to FIG. 1A of the accompanying drawings which illustrates the conveyance control mechanism of an ink jet recording apparatus according to the prior art, the reference numeral 51 designates a cut sheet which is a recording medium and which is conveyed in the direction of arrow after the writing timing in the sub-scanning direction is taken by register rollers 52.
The reference numeral 53 denotes paper keep rollers which limit the movement of the cut sheet 51 placed on a conveying belt 54. The reference numeral 55 designates a driving roller on which the conveying belt 54 is wound with predetermined tension. The reference numeral 56 denotes a charger which causes the cut sheet 51 on the conveying belt 54 to be electrostatically attracted to the conveying belt 54.
The reference numeral 57 designates a paper discharge tray onto which the cut sheet 51 after recording is discharged. The reference numerals 58-61 denote image buffers which memorize recording information data. Color data corresponding to various colors, i.e., yellow, magenta, cyan and black, are memorized in the image buffers 58-61, respectively, on the basis of a writing control signal from a controller 62. The controller 62 reads out the respective color data from the image buffers 58-61 at predetermined intervals after the register rollers 52 are driven, puts out the color data to recording heads 63-66, respectively, and records each color image on the cut sheet 51. The reference numerals 67-70 designate memory control lines which transfer the writing control signal from the controller 62 to the image buffers 58-61. The reference numerals 71, 73, 75 and 77 denote data lines which transfer the color data read out from the image buffers 58-61 to the recording heads 63-66. The reference numerals 72, 74, 76 and 78 designate recording control lines which transfer the recording timing signal output from the controller 62 to the recording heads 63-66.
The reference numeral 79 denotes a start signal output from a host, not shown.
The recording operation will now be described.
In a recording apparatus having a plurality of recording heads 63-66 thus disposed therein, when the cut sheet 51 is fed after the image recording timing in the sub-scanning direction is taken by the register rollers 52, the cut sheet 51 is attracted to the conveying belt 54 by means of the charger 56 and is conveyed. When together with this, a recording operation start command is output to the controller 62 by a start signal 79, image data (color data) is read out from the image buffer 61 to the recording head 66 which is a first recording head at a timing whereat recording is effected from the head of the cut sheet 51, and recording is started on the cut sheet 51 by the recording head 66.
Likewise, for the recording heads 65-63 which are second to fourth recording heads, a timing corresponding to the distance to the immediately preceding head is taken, and the image data read out from the image buffers 60-58 for respective colors are recorded on the cut sheet 51 by the recording heads 65-63 for respective colors, and as a result, a full color image is formed on the cut sheet 51, which is then discharged onto the paper discharge tray 57.
Now, on the conveying belt 54 for conveying the cut sheet 51 which is a recording medium, there may occur a periodic or non-periodic fluctuation in the conveyance speed of the cut sheet 51 as shown in FIG. 1B of the accompanying drawings, due to the irregularity of the thickness of the belt created in the manufacturing process thereof and the irregularity of the circularity of the driving roller 55 or the fluctuation of the driving load thereof.
FIG. 1B is a graph illustrating the speed irregularity characteristic of the conveying belt 54, and in this graph, the ordinate represents the belt speed and the abscissa represents time.
In this figure, I indicates the speed curve, T indicates a period corresponding to one round of the belt, and H indicates the maximum amount of speed fluctuation.
As can be seen from this figure, the speed of the conveying belt 54 shifts to the plus side (acceleration) and the minus side (deceleration) with respect to the normal standard conveyance speed V.sub.0, due to the irregularity of the thickness of the conveying belt 54 created in the manufacturing process thereof and the irregularity of the circularity of the driving roller 55 or the fluctuation of the driving load thereof. As a result, the time required from after the cut sheet 51 is conveyed from the register rollers 52 until the cut sheet 51 arrives between the recording heads 66-63 becomes irregular, and the writing timing of each of the recording heads 66-63 deviates thus causing density irregularity and misregistration of the image.
Particularly in the case of a color image, any minute color misregistration would cause the bleeding of colors, which in turn has led to the serious problem that the quality of the color image is remarkably deteriorated.
In order to solve this, in Japanese Patent Application No. 231469/1988 filed on Sep. 17, 1988, there is disclosed an apparatus in which the conveyance speed condition of conveying means for conveying a recording medium is detected and the recording timing of recording means is adjusted on the basis of the detected conveyance speed condition.
Also, U.S. application Ser. No. 501,499 (European Application Serial No. 90106169) discloses a non-contact Doppler velocimeter which is made compact as velocity detecting means and which does not cause any measurement error by the fluctuation of the wavelength of a light source, and discloses adjusting the recording timing of recording means on the basis of the detected conveyance speed condition. A velocimeter similar to the above-described non-contact Doppler velocimeter is disclosed in U.S. Pat. No. 4,948,257. Further, Japanese Laid-Open Patent Application No. 5260/1983 discloses the technique of finding discharge speed from the passage time of a liquid droplet passing between two points and achieving the stabilization of discharge.